The invention relates generally to mufflers of the type used with internal combustion engines to attenuate engine exhaust noise and, more particularly, mufflers conventionally referred to as xe2x80x9cside branch mufflersxe2x80x9d.
The invention is particularly applicable to and will be described with specific reference to a straight-through muffler for use in sports cars or high performance automotive vehicles. However, it will be appreciated by those skilled in the art that the inventive concepts disclosed herein may be utilized for any number of muffler applications and in combination with or as part of other muffler systems or arrangements for attenuating a specific frequency or a specific range of frequencies.
Engine noise from an internal combustion engine typically is generated by the sudden expansion of combustion chamber gases released from a combustion chamber. As the combustion gases are released and exhausted from each cylinder of the engine, a sound wave front travels at rapid sonic velocities through the exhaust system. This wave front is the boundary between the high pressure exhaust pulse and ambient pressure. When the sound wave front exits the exhaust system, it continues to pass through the air until three dimensional diffusion causes it to eventually dissipate. As the wave front passes an object an over pressure is created at the surface of the object and it is this over pressure that is a direct cause of audible and objectionable noise.
Since the inception of the internal combustion engine, efforts have been underway to reduce or muffle the noise caused by the engine. Obviously, considerable noise attenuation or reduction can be achieved in a muffler having dimensions that are large enough to permit three dimensional dissipation of the sound waves within the muffler housing. However, from a practical standpoint, design criteria often dictate the size of the muffler which typically must be kept as small as possible. Further means of attenuating engine noise include the use of packing and complex baffle systems. However, these approaches are often accompanied by a substantial increase in the back pressure or resistance of the muffler to freely discharge the combustion gases. The increase in back pressure can result in a decrease of the output horsepower of the engine with a resulting loss of efficiency in fuel economy.
Mufflers are classified in various manners within the art. From a structural consideration, mufflers have been classified as being either of two basic types or configurations:
1. A compartmentalized type which comprises several compartments sealed except for the inlets and outlets, the compartments usually-being sealed noise entrapment chambers; or
2. A type commonly known as a straight-through muffler which usually comprises a duct having a series of perforations within a sealed housing.
In accordance with this classification, the invention is particularly adaptable to mufflers of the straight through type, although it can have application to compartmentalized type mufflers.
From a functional view, mufflers may be classified as dissipative or reactive. Dissipative mufflers are typically composed of ducts or chambers filled with acoustic absorbing materials such as fiberglass, steel wool, or porous ceramics. Such materials absorb acoustic energy and transform it into thermal energy. Reactive mufflers, on the other hand, are composed of a number of interconnected chambers of various sizes and shapes in which sound waves are reflected to dampen or attenuate waves of a set frequency, typically resonance frequency. This invention relates to.a reactive type muffler.
There are two types of reactive mufflers. A side branch type muffler and a resonator type muffler. A resonator type muffler uses various volumes of different shapes or sizes, i.e. resonance chambers interconnected with pipes and can dampen not only resonance frequency, but also sound waves having frequencies near the resonance frequency. The drawback to resonator mufflers is the large volume required to dampen low frequency sound waves.
The side branch muffler is the type of muffler to which the present invention relates. Generally, the side branch muffler has a straight through pipe and an offset or side branching off the straight through pipe. The side branch pipe is closed at its end and may be bent or shaped with baffles. My U.S. Pat. Nos. 5,952,625 and 6,199,658 disclose a multi-fold side branch muffler that has advanced the art of side branch mufflers and such patents are incorporated herein by reference in their entireties.
When the sound wave front reaches the closed end of the side branch, it reflects back towards the open end thereby damping waves at the same frequency and out of phase with the reflected wave. Closed side branch mufflers such as disclosed in my aforementioned U.S. patents, have the limitation that they attenuate the fundamental and the odd harmonics of an objectionable frequency, but are not effective to reduce all of the harmonics. It has been found that particular engines and/or engine and vehicle combinations have noise characteristics in which the fundamental and all of the harmonics, or at least all of those near the fundamental frequency, are of consequence, i.e. loud.
Apart from the functional and structural considerations discussed above, sports cars and high performance vehicles have additional requirements. It has long been known that the exhaust systems of such vehicles must be tuned to emit certain sounds from the automobile which appeal to the purchaser of such vehicles, all while satisfying noise regulations or standards. Such applications require attenuation of specific waves having set frequencies to produce the desired sound. More particularly, high performance mufflers of the type under discussion are tuned to the specific type of engine to which the muffler will be applied. Specifically, the valving or breathing characteristics of the engine are matched to the muffler over the operating range of the engine to produce the desired tone. Recent engineering advances in the structural rigidity of the body or chassis of the vehicle in which the engine is mounted have enhanced the sound of the engine within the cabin or passenger compartment of the vehicle. Specifically, a muffler can be tuned to meet a desired sound with the engine on a test stand, but the muffler can produce objectionable resonance in the cabin. Since the cabin typically cannot be dampened, the muffler has to be precisely tuned to attenuate the sound waves producing the objectionable resonance within the cabin.
The invention affords a remarkably simple but surprisingly effective side branch muffler system that operates to reduce noise at a design frequency or limited band of frequencies and all of the harmonics. The invention resides in the provision of a side branch on a main exhaust pipe that is open at both ends and that has a length selected to cancel a particular design frequency. The side branch can have innumerable configurations but most typically runs parallel to the main exhaust pipe. As disclosed, the side branch pipe can lie alongside the main pipe or can be concentric with it. In either of these arrangements and in others where physical or economic constraints exist, for example, the side branch pipe can be folded on itself to reduce the length of its physical package.
When the invention is applied to passenger vehicles, for example, the muffler can be tuned so that it is most effective at the dominant resonant frequency in the passenger compartment. A muffler constructed in accordance with the invention is especially useful where increased power and/or fuel efficiency is of particular concern since it reduces back pressure in the exhaust system compared to more conventional muffler arrangements.